Apparatus for effecting heat exchange between a gas and a liquid



Jan. 1948- T. WOODWARD ET AL 2,434,976.

APPARATUS FOR EFFECTING HEAT EXCHANGE BETWEEN A GAS AND A LIQUID FiledOct. 2, 1944 s Sheets-Sheet 1 5' O E'Jil: II N C lclllllu III Ia lullINVENTORS 72-- Y/Vf/AM 14 000 WA R0 .AATHUR W VE'TTEL HEN/er FI. 504N0257'7 BYWG?@,W

Jan. 27, 1948. WOODWARD ET Al. 2,434,976

APPARATUS FOR EFFECTING HEAT EXCHANGE BETWEEN A'GAS AND A LIQUID FiledOct. 2, 1944 s Sheets-Sheet 2 Ffgfj A Z5 1 A k A A 7 2e 20 f t I 28 IINVENTORJ TEY/vHAM l l aaown 1P0 Aer/10,? W Verna. HENEYF ScmvoesrrJain. 27, 1948, r. WOQODWAVRLD ETAL 2,434,976

APPARATUS FOR EFFECTING HEAT EXCHANGE BETwEEN A GAS AND A LIQUID FiledOct. 2,. 1944 3 Sheets-Sheet b v mmvmm ZEY/W/AM W AR AETHUR W VETTEL 6fisxve Y F SCANORETT Patented Jan. 27, 1948 APPARATUS FOR EFFECTING HEATEX- CHANGE BETWEEN A GAS AND A LIQUID Teynham Woodward, Del Monte,Arthur W. Vettel, Watsonville, and Henry F. Scandrett, Oakland, alii.,asslgnors, by mesne assignments, to The Permanente Metals Corporation,

a corporation of Delaware Application October 2, 1944, Serial No.556,756

Claims. (CL 261-121) This application relates to an apparatus foreffecting heat exchange between a gas and a liquid, and particularly toeflecting exchange of heat by bringing into direct contact with eachother the two mediums between which exchangev takes place.

. The invention herein disclosed has very general application in that itis readily adaptable to use with many combinations of immiscible fluidsbetween which an exchange of heat is desired. It is, however,particularly adapted toone step of a process for manufacturing magnesia,or magnesium hydroxide, and will be described herein as so used thoughthis description is for the purpose of illustrating the invention and isnot to be taken as limiting the invention to the particular use selectedfor the purpose of illustration.

In the production of magnesium hydroxide, by the reaction of a liquidcontaining convertible magnesium salts, such as seawater, brines or thelike, and a lime-containing material, such as dolo- ,mite or the like,the salt-containing liquidis brought into contact with the lime therebyforming a sludge of magnesium hydroxide which is recoverable byfiltration. In this way, the magnesium hydroxide can be separated fromthe other constituents of the reacted mixture. However, the filtrationof this finely divided sludge material is a slow and rather expensiveoperation.

The rate of filtration and the capacity of filter equipment can beincreased by heating the material to be filtered. Advantageously. theliquid, such as sludge material, can be heated with stack gases,particularly those resulting from the calcining of magnesium hydroxidefilter cake. Stack gases are a convenient and inexpensive source of heatand serve to raise the temperature of the sludge material andsimultaneously to bring into contact therewith carbon dioxide gas.Carbon dioxide when thus added to the sludge material in small amountsalso has a beneficial effect upon the filtration rate.

It is therefore an object of this invention to provide an apparatus foreflecting eficient exchange of heat between a gas and a liquid bybringing the gas into direct contact with the liquid. It is anotherobject to provide an apparatus for introducing into a magnesiumhydroxide sludge which is to be filtered the proper amount of carbondioxide gas to facilitate filtration thereof. Other and more specificobjects of the invention are made apparent in the followingspecification wherein a preferred form thereof is gases issue from theoperation wherein a mag-,

described in detail and with reference to the accompanying drawings.

The term liquid as used herein is intended to mean any liquid-likesubstances including sludges, slurries, and other suspensions of solidsin liquid.

In treating magnesium hydroxide sludge to increase fllterabilitythereof, as one application of the process of this invention, heatedgases containing carbon dioxide are brought into contact with thesludge. When the heated gases are stack gases from sludge calcining,they generally contain about 15% of C02 on a dry basis. These nesiumhydroxide sludge is calcined by direct contact with gases obtained bythe combustion of fuel oil. but other hot gasesare useful and it isadvantageous to have a considerable conent of water vapor and arelatively low carbon dioxide content.

The sludge is usually heated to a temperature below that of the dewpointof the stack gases, which is about 70 C. in the case of gases fromsludge calcining as described above, but is preferably heated to about50 C. to attain the optimum effect of heating and of CO2 absorption. Inthis method, the conversion of magnesia to the carbonate is avoided.Only sumcient carbon dioxide, for example. up to about two grams perliter of sludge, is absorbed to facilitate filtration,

, without effecting significant reaction with the magnesium hydroxide.The solids content of the filter cake obtained by the filtration of thesludge shows a slight decrease. indicating that some of the magnesiumhydroxide, possibly the finest particles of the sludge, are carbonated,but this effeet is maintained at a minimum by this process. It isbelieved that the presence of the small amounts of carbon dioxide addedby this method may change the viscosity of the liquid to permit morerapid filtration and separation, but the mechanism of the action is notprecisely known.

It is an advantage of this process, in addition to the advantages shownabove, that carbon dioxide addition efiects. the recovery of a more,

highly purified magnesium hydroxide sludge.

The fine particles of magnesium hydroxide. formed by precipitationthereof in an aqueous medium are highly adsorbent, for example, to-

ward ions and therefore take up, for instance, chloride and sulfateions. It has heretofore been impossible to removethese absorbed ionsby'wa-sh- I .ing by the processes known to the art. It has now upwardlyin a diffused state 3 fects desorption of these ions and the finalwashed filter cake recovered by filtering the sludge and washing withwater is highly purified with respect to these adsorbed substances.

As an example of the process of this invention, with respect to treatingmagnesium "hydroxide sludge. seawater is brought into contact withcalcined dolomite to produce a magnesium hydroxide precipitate by amethod well-known to the art. The magnesium hydroxide sludge is washedby counter-current decantation and is then treated with stack gases in aprefer-red form of apparatus as more fully described below withreference to the drawings, and the treated sludge is finally filtered torecover a magnesium hydroxide filter cake, which may then be processedfurther as desired.

In the drawings:

Figure 1 is a vertical longitudinal section taken through an apparatusembodying the present invention. Figure 2 is a plan view of the sameapparatus with a portion of the top broken away. Figure 3 is atransverse sectional view taken'on line III-III of Figure 1 and Figure 4is an enlarged isometric view of one of the gas dispensers which form apart of the apparatus shown in the other figures oi the drawings. Figure5 is a schematic drawing of a multiple-stage heat exchanger according tothis invention.

Referring to the drawings in detail a large rectangular tank Ill isshown as a treating chamber. This tank is preferably of substantiallygas-tight construction and may be made either of metal orsuitably bracedwood such as heavy redwood planks. The tank, in the process specificallydescribed is usually under a vacuum of about five to seven inches ofwater. Sludge to be treated is passed through the tank continuously tomaintain a liquid level therein several inches above the bottom. Thesludge enters through pipes H disposed centrally and toward the ends ofthe bottom of the tank and flows toward the stand pipes l2, locatedabout midway between the ends and the central portion of the bottom ofthe tank. The height of the pipes l2 determines the liquid level in thetank. As the level of the liquid is a controlling factor in the treatingprocess, it is preferable that the height of the stand pipes beadjustable as, .for example, by couplings l3 threaded to the ends of thepipes or other means capable of manipulation to vary the 'eflectiveheight of the stand pipes through which the liquid sludge overflows.

It is possible to reverse or otherwise change the positions of thesludge inlet pipes and stand pipes, but it is preferred for maximumefficiency of contact that the fiow of sludge be maintained evenly overthe surface of the tank bottom. The arrangement shown here enables sucheffect, although other means may be employed to obtain such even flow.

As the sludge flows through the tank the hot gas which, in the presentexample, is stack gas from the operation of calcining magnesiumhydroxide filter cake and which is to be used to heat and otherwisetreat the sludge'by direct contact therewith, is admitted adjacent thebottom of the tank and is caused to bubble or pass through the sludge.It is desirable that the gas in passing through the liquid be spreadthroughout substantially the entire area of the tank, that it be finelydiifused or .broken up to efi'ect a large area of surface contactbetween the fluids, and-that it pass through persers l4,

aison 4 the liquid sludge to produce a uniform ebullient action ratherthan the relatively high state of agitation that would result fromdirecting the gas through the liquid by means of small Jets 0! underhigh pressure. These requirements are met by the apparatus disclosed.

The apparatus comprises a plurality of inverted trough shaped gasdispensers H, the construction of which is best shown in Figure 4 of thedrawings. The gas dispensers 14 extend across the tank in a common planenear its bottom and each of them comprises an inverted channel or troughwhich may be formed of sheet metal or the like and which is welded orotherwise suitably secured to a pipe l5, which, as best shown in Figures2 and 3, extends through the wall of the tank I 0 and connects with amanifold l8 supplied with gas through a conduit I I which conduitconnects with the stack, not shown, from which the gas is drawn. A valveor damper I8 is advantageously placed in the discharge line and may beused for the purpose or controlling the amount of gas travellingtherethrough. The pipes II are connected with the manifold asillustrated in Figure 3 through connecting pipes l9 and-the ends of thepipes l5 are continued outwardly and terminate in removable caps, suchas indicated at l0, to facilitate the removal of dust and carbon whichmay be excessively deposited in them by the stack gases. Through thisarrangement it is possible to inspect and clean the interior of thepipes IS without interrupting the continuous operation of the apparatuswhere the pipes l5 penetrate the wall of the tank l0. Ordinarily thedust is prevented from depositing in the pipes by a water-spray which iscontinuously drawn into the pipes by the Venturi eirect oi the stackgases passing therethrough. Pipes 28 project upwardly into the ends ofpipes I 5, and through the walls thereof, conduct small streams of waterinto pipes i5. By the action of the stack gases these streams areconverted into water sprays which eifectively wash away the dusts fromthe interior walls of the pipes, thereby reducing the necessity formanual cleaning and also preventing corrosion of the pipes by thematerials in contact with them. The pipes are, of course, sealed againstleakage of the contents of the tank and in the event the tank is made ofwood some heat insulation such as asbestos packing, not shown, ispreferably employed to separate the pipes from contact with the wood soas to prevent charring oi the latter due to its continuous exposure tothe heat of the gas carried by thepipes. Referring again to the gasdispersing'members M, as illustrated in Figure 4, the lower edges of theinverted channels which form these members are serrated to provide aseries of inverted V-shaped notches 2| which cause the gas to escape instreams of small bubbles and to pass upwardly through the sludge in afinely divided or diffused state. That end of the channel opposite toits point of connection with the pipe I5 is closed. In an alternativeconstruction, the gas disperser may extend through the wall of the tankopposite the entry and be suitably sealed or! with a removable cap whichcan be taken oil. to permit inspection or cleaning. Preferably fromthree to five gas dispersers are disposed in the path of flow between aninlet zone and the corresponding outlet zone.

Blocks 21 are placed between parallel gas dissuitably in a staggeredrelationship as shown in Figure 2. and act to prevent surging a ore orwave formation along the sludge body. The flow of sludge should bemaintained evenly, as stated above, by reason oi the circumstance thatgases passing therethrough will tend to take the shortest path throughthe sludge, that is, they will pass through the sludge and issuetherefrom in the trough of the wave. effecting shorter contact time andlowered efiiciency. The blocks 21 prevent or break up the development ofwaves and enable increased eficiency of contact.

The treated sludge is drawn on through stand pipes it and passes into atra 29 and thence to a storage tank not shown from which it is rapidlypumped to the filters.

The gas which enters through the dispersers and passes upwardly throughthe sludge into the I upper portion of the tank in is drawn oil througha conduit 22 by a suction fan 23 driven through a pulley it by a motoror the like, not shown. Preferably a spray trap 25 and baille d2 areinterposed between the tank and the gas discharge conduit 22 to preventcarry over of spray to the fan and build up of any deposit. The gasesreturn to the stack, or other point of discharge, through an exhaustconduit 29 of the suction fan 28. While it is apparent that the suctionfan in the gas discharge conduit could be replaced by a blower at thefeed end the arrangement shown is preferred in that the gas handled bythe fan is considerably cooler after its contact with sludge.- In theparticular process here described, the stack gas enters the sludge at atemperature of about 250 to 300 C. and is discharged from the treatingtank at a temperature of about 50 to 60 C. Furthermore, the volume ofgas handled by the fan at the discharge end is considerably less becausea, large part of the available heat in such stack gases is due to thelatent heat of water vapor which they contain. Condensation of thisvapor to liquid in the sludge greatly reduces the volume of the gas.

One of the advantages of the method and apparatus herein describedresides in the fact that the gas passes through a relatively small bodyof sludge so that the pressure drop throu h the apparatus is relativelysmall but the efliclency of heat exchange is exceedingly high. Inpractice the level of the sludge in the tank is maintained at about fiveor six inches above the bottom of the gas dispersers and under theconditions described it has been demonstrated that the temperature ofthe gases leaving the heat exchange operation is ble through thepartially treated sludge on level and are drawn on at it and conductedto a similar series of gas dispersers on level it. The gases passthrough the fresh sludge, giving up residual heat thereto and are drawnoil at so and pass to the stack. The gases are drawn through the systemby pump iii also advantageously disnosed at the-outlet zone for thegases. The efflciency of the process is still further increased by themultiple stage exchanger which, although described with respect to atwo-stage device, may have more than two series of exchangers.

The apparatus and process of this invention are also useful in manyother applications than that described, as for example, in 'pre-heatingseawater in the seawater process for making magnesium hydroxide, or inpre-heating wash-water in other processes, such as in mining operationsand the like and are not intended to be, limited to the preparation ofany particular material.

It is an advantageous eflect of this invention that rate of filtrationand thereby the capacity of the filter equipment is increased, intreating difflculty fllterable materials. It is a further advantage ofthis invention that it provides a method and device whereby the dustcarried by the stack less than 2 C. above the temperature of the sludge.Thus, a very high degree of emciency has been obtained by the practiceof this invention with relatively simple mechanism constructed at a lowcost and capable of continuous operation. A deeper body of liquid may bemaintained, but as the depth increases the eiilciency of the processbecomes less and it is preferred to operate with a shallow body ofliquid.

In another embodiment of the method and apparatus of this invention, amultiple stage heat exchanger is employed. As an example of this, Figure5 shows a schematic diagram of such a device. In this figure, atwo-stage heat exchanger is shown. The sludge t be treated is led in onthe upper level Bil through pipes 3i and flows across lit to the outletor stand pipe 32. Through 32 the sludge, partially treated, passes downto an overflow pan 33 and then is led out by conduits St to lower level35, over which. it flows to outlet or stand pipe to whence it is removedand goes to the filters. Stack gases are led in through a manifold Bl,pass through gas dispersers 3d, bubgases is removed therefrom. Stackgases from the calination of filter cake as described above contain asignificant amount of MgO dust particles. This MgO dust is recoveredfrom the gases by the process and apparatus of this invention, therebyreducing incidental losses of MgO, in the processing of magnesiumhydroxide sludge.

What is claimed is:

1. In an apparatus of the character described, a tank, means fordelivering a liquid into the central bottom portion thereof and into thebottom portions ad acent the ends of the tank, overflow stand pipesmidway between the pairs of the said central and-end inlet means tomaintain a shallow, uniformly distributed flow of liquid over the bottom01' the tank, channel gas dispersers having serrated lower edges anddisposed adjacent the bottom of the tank to admit gas into the body ofthe liquid, atleast one end of each of said gas dispersers terminatingoutside of said tank and being sealed oil by a removable cap, meansdisposed on the bottom of the tank between said gas dispersers toprevent wave formation in the flowing liquid, and means for maintaininga reduced pressure over the surface of the liquid in the tank to inducethe gas to flow through the liquid.

2. In an apparatus of the character described, a tank, means fordelivering liquid into the central bottom portion thereof and into thebottom portion adjacent the ends of the tank, overflow stand pipesmidway between the pairs of the said central and and inlet means tomaintain a shallow, uniformly distributed flow of liquid over the bottomof the tank, channel gas dispersers having serrated lower edges anddisposed adjacent the bottom of the tank to admit gas into the body ofthe liquid, at least one end of each of said gas dispersers terminatingoutside oi said tank and being sealed ed by a removable cap, meansdisposed on the bottom oi the tank between the gas dispersers to preventwave formation in the flow-- ing sludge, a gas exhaust conduit above theliq= uid level in the tank, a suction fan in said cons duit, and a spraytrap chamber interposed between said conduit and the interior of thetank.

3. In an apparatus of the character described, a tank, means fordelivering liquid into the central bottom portion thereofand into thebottom acaaova portions adjacent the ends of the tank. overflow standpipes midway between the pairs the said central and end inlet means tomaintain a shallow. uniformly distributed flow of liquid over the bottomor the tank, channel gas dispersers having serrated lower edges anddisposed adjacent the bottom oi the tank to admit gas into the body ofthe liquid, at least one end of each of said gas dispersers terminatingoutside or said tank and being sealed oil by a removable cap. blocksdisposed on thebottom of the tank between the gas disperser-s to preventwave formation in the flowing sludge. a gas exhaust conduit above theliquid level in the tank, and a suction fan in said conduit.

4. Anapparatus for edecting exchange of heat between stack gases andmagnesium hydroxide sludge which comprises a tank, means for maintaininga uniformly distributed flow of liquid at a constant level through saidtank, gas-admit- 2o ting means including channel gas dispersers havingserrated lower edges for introducing gas to within said sludge adjacentthe bottom of the tank, at least one end of each 01 said gas dispersersterminating .outside at said tank and being sealed oil by a removablecap, water-spray means adjacent to said gas-admitting means and actuatedby the gas flow for washing dust from the interior walls oi saidgas-admitting means into the liquid, and means for maintaining a re- '302,070,067

duced pressure over the surface or the liquid in the tank to induce thegas to flow through the liquid.

5. An apparatus for effecting exchange of heat between stack gases andmagnesium hydroxide sludge which comprises a tank, means for deliveringsaid sludge ,into the central bottom portion thereof and into the bottomportions adjacent the ends or the tank, overflow stand pipes midwaybetween the pairs 01 the said central and end inlet means to maintain ashallow, uniformly distributed flow of sludge over the bottom of thetank, gas-admitting means including channel as dispersers havingserrated lower edges and disposed adjacent the bottom of the tank toadmit said stack gases into the body of the liquid. at least one end ofeach of said gas dispersers terminating outside of said tank and beingsealed off by a removable cap, blocks disposed on the bottom of .thetank between saidgas dispersera to prevent ,wave formation in theflowing sludge, water-spray means adjacent to said gas-admitting meansand actuatedby the gas flow for washing dust from the interior walls ofsaid sasadmitting means into the sludge. a gas exhaust conduit above theliquid level in the tank. a suction tan in said conduit, and a spraytrap chamber interposed between said conduit and the interior oi thetank.

. TEYNHAM WOODWARD.

ARTHUR W. VETTEL. HENRY F. SCANDRE'I'I.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date Rice Feb. 9, 1987 1,236,581McGrath Aug, 14, 1917 1,639,709 Still Aug. 23, 1927 2,273,178 CollinsFeb. 17, 1942 2,231,965 V Stump Feb. 18, 1941 2,090,994 Brandes Aug. 24.1937 1,939,949 Bertram Dec. 19, 1933 1,098,190 Stone ..--l -1-.. May 26,1914 1,938,247 Collins Dec. 5, 1933 575,370 Parks Jan. 19, 1897 40889,694

Lambert June 2, 1908

